In a prior invention by applicant set forth in U.S. Pat. No. 3,649,393, which issued on Mar. 14, 1972 on an application filed June 12, 1970, and assigned to the same assignee as the instant application, a method is set forth for etching a thin metal film on an oxide which is not uniformly thick. Such thin film, which may be self-supporting or supported on a substrate, has markedly different thicknesses in different areas to be etched. A photoresist is placed over the surface of this film of non-uniform thickness and the photoresist covered, variably thick, film is placed in an evacuated chamber that houses an electron beam apparatus. The photoresist is exposed by an electron beam in a series of separate exposures with different exposure densities. The thickest area is exposed first with the highest exposure density. Subsequent exposures are made in the other desired areas with decreasing densities in accordance with decreasing thickness. In the developement steps, the photoresist is developed until the area of highest exposed density is opened and the thin film is etched to the next thickness level. Development is continued until the second highest exposed density is opened and then the thin film is etched to the next film level. Such steps are repeated for each succeeding film thickness until the minimum film thickness is uncovered.
X-rays are now desirable for lithographic fabrication because X-rays allow planar gold patterns on thin silicon substrates to be projected deep into resist material without significant absorption or spreading due to scattering or diffraction. Upon chemical development, one obtains large groove depth to period ratios which are necessary for high efficiency. Such advantageous uses of X-rays for lithography are discussed in greater detail in an article entitles "X-ray Lithography: Complementary Technique to Electron Beam Lithography", by H. I. Smith et al., Twelfth Symposium on Electron, Ion, Laser Beam Technology, MIT, Cambridge, Mass., May 21-23, 1973.
In my prior patent U.S. Pat. No. 3,649,393, a stepped oxide or stepped metal layer (layer 12 in such issued U.S. Pat. No. 3,649,393) results from certain needed fabrication steps employed in the manufacture of semiconductor chips. Due to previous steps, such as the making of openings in an oxide layer over silicon so as to permit diffusion of ions or the like into the silicon through such openings, such graduated layer results. The small openings are reoxidized to heights that are less than the original oxide layer. When such steps are repeated, one attains regions of different oxide thicknesses. My prior patent teaches etching steps in the fabrication process and as such tends to diminish the chances of obtaining high device resolution. In general, one would like the spacing between thick regions to be equal to or less than the height of the thick regions. But due to the undercutting that results when etching is employed, such spacing is greater than the height of a region between spacings and is often about twice that of the height.